It's not exactly rocket science. Television receivers are superhets (and therefore contain a local oscillator). Some of the LO signal couples into the antenna cable. Television antennas are highly directional, and have just enough bandwidth to transmit at the LO frequency.

Now, if the detector van's receiver was a superhet -- and, since this is the most sensitive kind of receiver, it's unlikely to be anything else -- then you could theoretically build a TV detector van detector, which would cut off the power to your TV when it detected the detector van's own local oscillator! But in order to do this, you would need to know the intermediate frequency used by the detector van's receiver, in order to work out the local oscillator frequency that you were looking for.

What I want to know is, WHY did the Government not mandate card readers on ALL digital receivers from the beginning of the switchover? Then the BBC could have broadcast their programmes scrambled, and anyone without a viewing card would also be without pictures. That would surely have been the fairest way to do it. It's inconceivable that nobody from the industry would have mentioned this possibility during the initial consultation, so on what grounds was it blocked?